Environmental stressors, such as drought, water-deficit, high light, and extreme temperatures, represent a few of the major factors that affect plant growth, survival, and productivity. Droughts, transfer of plants from greenhouse to field, and extreme temperature and light fluctuations can be devastating to crop and ornamental plants, resulting in the loss of billions of dollars. Breeding of stress-tolerant crops is one approach to these problems, but conventional breeding is a slow process for generating plant varieties with improved tolerance to stress conditions. Limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species are additional problems encountered in conventional breeding. Recent progress in plant genetic transformation and the availability of potentially useful genes characterized from different sources have made it possible to generate stress-tolerant crops using transgenic approaches.
In most photosynthetic organisms, the conversion of light energy to chemical energy occurs largely through linear electron transport (LET), which generates both the NADPH and the ATP required for CO2 assimilation. However, electrons can also transfer cyclically around PSI from ferredoxin (Fd) or NADPH back to the plastoquinone pool (PQ), thus generating ATP via cyclic electron transport (CET). In cyanobacteria, green algae, and the bundle sheath cells of C4 plants, CET can be the exclusive source of photosynthetically-derived ATP, or it can act to provide extra ATP for different cellular processes. In C3 plants, the importance of CET has been the subject of much debate; it has been proposed to play a key role in photoprotection by mediating nonphotochemical quenching (NPQ) and stromal redox status, and alternatively to generate ATP essential for the Calvin Cycle and other metabolic processes.
Two CET pathways are known to exist in C3 plants. In one of them, electron transfer from Fd to PQ requires proton gradient regulation 5 (PGR5). PGR5 is a small, nucleus-encoded, thylakoid membrane-associated protein. Research suggests that PGR5-dependent CET acts to increase thylakoid lumen pH (ΔpH) under photoinhibitory conditions by facilitating the movement of electrons from NADPH or Fd to the PQ pool (Munekage et al., 2002). In the second CET pathway, electron transfer from NADPH to the PQ pool is mediated by NAD(P)H dehydrogenase (NDH). NDH is a large, partly chloroplast-encoded, thylakoid membrane-bound, multi-subunit complex. Research suggests that the two CET pathways act in a partially redundant manner to generate the ATP needed for proper growth (Munekage et al., 2004).